the story is rather simple - it refers to how WGBT is determined and
what is supported by them.
The general formula for determining WBGT outdoors is:
WGBT = 0.7 x T_nwb + 0.2 x T_g + 0.1 x T_a
where nbw is natural wet bulb, g is BGT and a is outdoor temperature
- to get T_a in an Ecowitt context, you need an outdoor T&H sensor (e.g.
WH32)
- to get T_g in an Ecowitt context, you need a WN38 Black Globe
Temperature sensor
- to get T_nwb there is no sensor provided - therefore an approximation
method is being used
only in this combination Ecowitt provides WBGT data
and as WBGT has a medical dimension, the disclaimer is on there in their
(Ecowitt) web shop
there are basically two approximation methods for determining T_nwb and
T_g
1. the so-called Stull formula which uses temperature, humidity and air
pressure (and derived entities like water saturation pressure)
2. the so-called Lijlegren approximation which also uses wind speed and,
depending on the accuracy targeted, also sometimes solar radiation
values (direct, indirect, at different wave-lengths ...)
1. the Stull approximation implemented in the firmware now (and used by
many others) can lead to a deviation of up to 3° C
2. is quite a sophisticated approach with several iterations, depending
on if the approach is used for T_nwb only or for T_nwb + T_g and if
solar values are included or not
to program the full Lijegren approach to reach a < 0.1% deviation from a
nbw-sensor, a coding of 300 - 400 lines of code is needed due to
repetetive iterations (so 'they' say)_
- for so-called "medical" (health related) purposes, a simplified
Lijlegren approach can be used also using wind speed and it produces a
deviation of < 0.3° C - that's sufficient for the WGBT tiers/windows
used in different areas
- Ecowitt is considering to replace the Stull formula used now by a
simplified Llijlegren approach including wind speed to reach a higher
degree of accuracy
so, the accuracy of WBGT values provided by any piece of weather station
firmware or data logging software (like weewx) will depend on the
approach used.
On 23.06.2026 20:38, David Hathaway wrote:
Thanks for the discussion pointer. I'll go snoop.
On Tuesday, June 23, 2026 at 1:09:37 PM UTC-5 Vince Skahan wrote:
I recently saw mention of https://shop.ecowitt.com/products/wn38
which has a disclaimer that I didn't look into further.
/"The WN38 Black Globe Thermometer measures and displays the black
globe temperature only, which can be viewed on the LCD screen or
through the Ecowitt or WS View Plus app./
/To display WBGT (Wet Bulb Globe Temperature) data in the app, the
WN38 must be used together with an Ecowitt outdoor temperature and
humidity sensor from either an outdoor array or a WN32(WN31 is not
feasible)."/
There is a lot of discussion at
https://www.wxforum.net/index.php?topic=49316.0 which might be
worth a read. Hope this helps./
/
On Wednesday, June 10, 2026 at 10:03:28 AM UTC-7 David Hathaway wrote:
There is a 'wetbulp.py' (note the p) in this group that
calculates it as a xtype_service. This conversation,
https://groups.google.com/g/weewx-user/c/FntQukhWY00/m/lM53yFt7AwAJ
On Wednesday, June 10, 2026 at 11:48:33 AM UTC-5 Vince Skahan
wrote:
Quick search seems to say a WeatherFlow Tempest station
has this, but I can't find it mentioned in any of their
API so I don't see a way to get it into weewx for this one
model station -
https://community.tempest.earth/t/wet-bulb-globe-temperature/14386/11
Other threads and docs with the formula seem to say you
need special sensors for this, so I can't speculate how
WeatherFlow fakes something to generate that item.
On Wednesday, June 10, 2026 at 8:36:45 AM UTC-7 David
Hathaway wrote:
As the FIFA World Cup (tm) ramps up, we here in
Houston are getting a load of notices about how there
is a mandatory water break in matches, even though it
is played indoors. :-/ As I discussed this with my
friend the sports trainer, he mentioned that Texas now
has a rule that they must do certain things according
to the WBGT.
I have searched here for wet bulb additions to WeeWx.
I see discussions from 2024 and wonder if there is
someone with a more recent idea about implementing
this. Does anyone calculate the Risk Level and
provide Guidance?
I am trying out an agentic Ai to read the RSS XML
feed; It loves this format. I've not messed with the
RSS export. Is there some intrisic gotcha? I really
only care about current values.
Dave
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